A file system–internal data structure that assures all the data on disk is properly organized and accessible. Essentially, it is “data about the data.” Almost every file system has its own structure of metadata, which is partly why the file systems show different performance characteristics. It is of major importance to maintain metadata intact, because otherwise all data on the file system could become inaccessible.

Inodes contain various information about a file, including size, number of links, date and time of creation, modification, and access, as well as pointers to the disk blocks where the file contents are actually stored.

In the context of a file system, a journal is an on-disk structure containing a kind of log in which the file system stores what it is about to change in the file system's metadata. Journaling greatly reduces the recovery time of a Linux system because it obsoletes the lengthy search process that checks the whole file system at system start-up. Instead, only the journal is replayed.